This would be really useful when, for instance, doing welds.
I'm welding a large number of angle iron backing pieces to a steel plate. I click on "1" for the first selection set, then click the faces on all the angles, then click "2" for the second selection set and click the plate. Then I notice that - ah, cr*p - I missed a couple of faces in the first set, so click "1" again, go and fix those, then I realize that I can actually take care of a couple of other things here too, so I click "2" again, and ... etc.
It would be really great if I could avoid some of the clicking around in the dialog boxes by just holding down ALT or SHIFT or CTRL or something in order to pick faces for the second selection set. This would probably save me anywhere between 10 and 30 minutes over the course of a day.